Power converters such as switch-mode (switching) power converters are widely used in various electronic devices for sourcing power to the electronic devices from a power source. Taking a flyback type switching regulator for example, the flyback switching regulator is generally applied in applications where the output should be insolated from the input, and the flyback switching regulator can provide one or more isolated output voltages.
A power converter such as a flyback switching regulator usually comprises a control circuit for controlling a switch module to switch on and off so as to convert an input voltage to an output voltage. The switching of the switch module generates a switching current. The percentage of the on time of the switch module in a switching cycle is referred to as a duty cycle. The control circuit can use various control schemes/control modes for controlling the switching regulator. One of the most generally used control modes includes peak current control pulse width modulated mode. In brief, under the peak current control pulse width modulated mode, the control circuit provides a difference signal indicative of a difference between a feedback signal indicative of the output voltage and a reference signal indicative of an desired value of the output voltage, and compares a current sense signal indicative of the switching current with the difference signal to provide a pulse width modulated signal to control the on and off switching of the switch module. For preventing or reducing sub-harmonic vibrations of the switching regulator system, the current sense signal is often compensated with a ramp waveform before being compared with the difference signal.
For the operation safety of the switching regulator, it is also desired to limit the output current or the switching current so that the output current is lower than an allowable threshold of the output current or the switching current is lower than an allowable threshold of the switching current. In practical application, the limitation to the switching current or to the output current can be implemented through limiting the peak value of the switching current to be lower than a predetermined peak current threshold. This is often realized by comparing the ramp compensated current sense signal with the predetermined peak current threshold so as to prevent or reduce the problem of sub-harmonic vibrations.
However, although compensating the current sense signal with the ramp waveform can help to resolve the problem of sub-harmonic vibrations, this may result in decrease in the maximum allowable output power of the switching regulator with the increase of the duty cycle, which is undesirable.